Thermoelectric device



Patented Apr. 2, 1946 No Drawing. Application December 24, 1942, se i'ilal No. 470,032. In Great Britain July 2,

3 Claims.

This invention consists of improvements in or relating to thermo-electric devices. Many different proposals have been made for producing suitable elements for thermocouples and in particular for'obtaining a high degree of sensitivity in thermopiles. It is known that to obtain a high sensitivity it is necessary that the substances used must possess a high thermo-electric power and that the loss of heat from the irradiated surface must be kept as small as possible. To minimize heat loss it has been customary to use the thermo-electric elements in the form of fine wires or strips sometimes joined end to end but these arrangements are delicate and are apt to be injured by shockor vibration. When attempts have been made to construct these fine or thin elements from metals or compounds of high thermo-electric power like metallic tellurium or bismuth or antimony or their alloys they are found to be very brittle so that they are diflicult to handle and do not effectively resist shock and vibration.

One object of this invention is to provide compositions suitable for thermo-electric elements of high and variable thermo-electric power and low heat conductivity which shall be strong and tough and readily worked.

The conditions which these compositions should fulfil are that they should have the following properties:

(1) High and reliable thermo-electric power capable of variation.

(2) Low specific resistance. (3) Low heat conductivity. (4) Good mechanical properties. (5) Appropriate melting point neither too low nor too high (e. g. between 500 C. and 800 C.). 6) Resistance to oxidation (on heating in air). (7) Weldability to metals such as silver or gold. (8) Stability either in air or vacuum. (9) In certain cases, as in pyrometers, a low rate of change of resistance with temperature.

According to this invention a composition is made from a metal or from two or more metals combined with two or more elements of the sulphur group.

All the conditions mentioned above are fulfilled if the composition consists of one, two or three metals with elements of the sulphur group. The convenient metals are silver, copper, lead and thallium and the elements of the sulphur group are sulphur, selenium and tellurium.

It is to be understood that if both thermoelectric elements contain two or more metals combined with two or more elements of the sulphur group the thermo-electric effect is secured by arranging that in one thermo-el-ctric composition the proportion of a certain metal or compound (or of certain metals 'or compounds), is comparatively high whereas in the other thermo-electric composition the proportion of the constituent metals and elements is substantially difierent. By alteration of the proportions it is possible to fulfil all the nine conditions set out above.

A metal such as silver may be melted with sulphur and selenium to form a composition for use as one element in a thermo-electric device. This composition may contain minor proportions of other metals such as copper or lead and/or tellurium. For example, one element herein referred to as the positive element may have a composition as follows:

Per cent Copper 1 Silver 78 Tellurium 0.6 Selenium 1.3 Sulphur It is found that by varying the proportions of the elements in the above composition, the thermo-electric power against platinum can be varied within a range of about 130 to 1200 microvolts per degree centigrade. An increase of sulphur and reduction in selenium increases the thermo-power. Increase in tellurium decreases the specific resistance and the rate of change of resistance with temperature. Thus in producing a positive element the amount of copper is preferably less than 2%, the amount of silver may range between 70 and 80%, the amount of selenium may range between 10 and 15%, the amount of sulphur may range between 5 and 10%, and the amount of tellurium is preferably less than Copper and silver and tellurium with selenium and sulphur are melted together to form a composition for use as another element of a thermoelectric device (say the negative element) and the following is one example of an appropriate composition, the figures being given as percentages by weight:

Copper 27 Silver 32 Tellurium 33 Selenium v 7 Sulphur 1 proportions may increase the thermo-power of' thepreferred arrangement the bodies above referred to are conductively connected, say by electric welding. to the supporting pillars and to the foil orreceiver.

This invention can be applied to the following uses:

V 1. In thermopiles for detecting and measuring 7 radiation:

said compositions and may also improve the'mechanical properties by making the bodies tougher, more ductile or harder. Thallium may also be added. A, simple test may be necessary to determine the appropriate proportion of such additions because their effect varies with the composition of the remainder. V 7

These compositions are not brittle but quite tough and they can be made into suitable forms for securing, say'by welding, toconducting supports and to irradiation surfaces. V

The present invention can be used in conjunction with that described in my United States patent application Serial No. 470,031 filed December 24, 1942, that is to say, the two compositions may be used in the form of little sturdy bodies (say conical or cylindrical bodies) secured to the outer ends of supporting pillars of conducting material and the tips of these bodies may be secured to the back of a metallic foil the outer sur-' face of which is the irradiation surface.

These bodies are semiconductors.of electricity and have a low heat conductivity. 7 V

The thermo-electric powers of the two bodies (in relation to gold or platinum) may be of opposite sign so that their thermo-electric effects are added when the deviceis in operation in a completed circuit. r

It will be understood'that for differenttypes of instruments involving the use of thermo-electric couples the conditions or requirements may vary and, as has been pointed out, the thermoular use of this invention-a simple test. may be necessary to determine the optimum composition it is a subsidiary feature of this invention that in- 2. In,thermo-converters for converting alternating to direct current;

3. In the transformation of radiation energy (say from the sun) into electric current;

4. For measuring degree of vacuum (as the sensitivity of the thermo-electric device increases as the gaseous pressure diminishes);

5. In electrical relays aflected by radiation as a in burglar alarms;

selection of the resistance and sensitivity of the thermopile. It is one of the practical advantages of the present invention that the resistance and the sensitivity of the thermopile can be independently varied to an outstanding extent.

I claim: r a

1. A thermo-electric couple having a positive thermo-electric element-comprising a fused material'containing between 70 and 80% silver, less than 2% copper,'10 to 15% selenium, 5 to 10% suliur, and less than 2% tellurium, and a negative thermo-electric element comprising a fused material containing 25 -to copper, 25 to silver, 30 to 35% tellurium, 5 to 10% selenium, and less than2% sulphur. 1

2. Apositive thermo-electric element comprising a fused material containing between to silver, less than 2% copper, 10 to 15% selenium.

' 5 to 10% sulphur, and less than 2% tellurium.

'3. A negative thermo-electric element comprising a fused material containing 25 to 30% copper, 25 to;35% silver, 30 to tellurium, 5 to 10% selenium. and less than 2% sulphur-r ERNST SCHWARZ. 

